My blur node does not affect a shadow between rendernodes
Fake way to do it is increase the size of the light, a lot.



Another fake way to do it is by mixing a transparent node with your shader and use facing layer weight for factor



(sorry for self hosting, imgur declined my request)

And now the real way: volumetrics



Plug a volume scatter shader to the volume output and control it's density with a spherical gradient. Probably can be modified for non spherical objects too.